The accuracy of integrated temperature sensors can potentially be limited by semiconductor process parameter variations. For example, the accuracy of bipolar-based temperature sensors may be limited by the base-emitter voltage (VBE) variations among the devices in a batch. Likewise, high accuracy CMOS temperature sensors are typically based on substrate PNP bipolar VBE and ΔVBE, and can be subject to similar variations. The accuracy of such temperature sensors, without calibration, is commonly about +/−2° C., over the temperature range −55° C. to +125° C. This means that for temperature sensors that are design-limited by VBE variations, calibration is desired for an accuracy of less than +/−2° C.
To calibrate a temperature sensor, a reference temperature with accuracy better than the temperature sensor is desired. The output of the temperature sensor can then be compared to the reference temperature for calibration purposes. Calibration schemes are generally either thermal calibration schemes or electrical calibration schemes. For example, one thermal scheme includes using a temperature bath or chamber to produce a reference temperature, and a high accuracy thermometer to measure the reference temperature. The measured reference is compared to the output of the temperature sensor to be calibrated. However, it can be difficult to both control the temperature of the reference chamber environment and to accurately measure it. Additionally, it can take a relatively long time (on the order of minutes or tens of minutes, for example) for thermal contact and stabilization. This increases the cost of the test stages of the manufacturing process of each part.
In an alternate thermal process, calibrating a wafer made up of multiple sensors can spread the overhead over the multiple sensors. Additionally, the average error of a batch of sensors can be measured, and each sensor of the batch can be calibrated based on the average error. However, group calibration does not address temperature errors (which can be on the order of +/−0.5° C.) that can be due to the mechanical stress effects of packaging. Further, group calibration avoids calibrating each sensor part, but the resultant accuracy depends on the accuracy of the average error (number of samples required), the intra-batch variation, and the reproducibility.
One electrical calibration scheme includes using electrical means to measure the reference temperature, for example. However, while an electrical measurement can be fast (seconds compared to minutes), the use of such high-precision test measurement equipment in a production test environment is not trivial. Alternately, an electrical means may be used to calibrate VBE indirectly by calibrating the bandgap reference voltage used in the analog-to-digital converter (ADC) of the sensor. For example, the precise temperature may not be as important, since the temperature coefficient of a trimmed bandgap can be zero.
On the other hand, the bandgap has a zero temperature coefficient only at the trim temperature with curvature over the temperature range. In addition, the target reference voltage for a zero temperature coefficient calibration has some dependency on the calibration temperature and other parameters (including the ideality factor η). For a certain type of curvature corrected temperature sensor, a non-zero temperature coefficient voltage reference is needed, which then requires the temperature to be known to determine the correct trim reference voltage. In addition, for a temperature sensor ADC that uses a charge balancing technique, the reference voltage is often generated dynamically from ΔVBE and VBE and is not available for direct measurement.